Oakland police took the unusual step Friday of showing pictures of the guns wielded by two people shot to death by officers over the past week - one a 71-year old man, the other a 15-year-old boy - and said officers had no choice to but to shoot after the weapons were pointed at them.

At about noon Wednesday, three officers from the police gang unit shot and killed 15-year-old Jose Luis Buenrostro, who allegedly has ties to a gang, after he pointed a sawed-off rifle at them near 79th Avenue and Rudsdale Street, said Oakland homicide Lt. Ersie Joyner.

At about 7 p.m. on March 14, an officer shot and killed Casper Banjo, 71, near the Eastmont police substation on 73rd Avenue after he pointed a replica pistol at officers, police said.

The separate shootings in East Oakland are being investigated by Oakland homicide and internal-affairs investigators and the Alameda County district attorney's office.

But preliminary investigations show that the officers acted appropriately in each case, opting to use deadly force because guns were being pointed at them, police said at a news conference Friday.

"It is a crime to point a gun at a police officer, and you can reasonably expect that bad things are going to happen," said Deputy Chief Jeff Israel. "And sometimes that means we discharge our firearms, and people are seriously hurt or killed."

Officers are trained to stop a threat by aiming at someone's torso - also called "center mass" - rather than trying to shoot at the arm or leg, which is the stuff of movies or TV crime shows, said Officer Roland Holmgren, department spokesman.

Holmgren showed pictures of the sawed-off rifle reportedly used by the teenager and the replica pistol that Banjo allegedly used. The replica pistol was compared to a real one in pictures.

"It's extremely hard to tell the difference between the two," Holmgren said, adding that officers often don't have time to make such a differentiation.

After the youth - a student at an Oakland charter school focused on aviation - and Banjo were shot, the weapons they had been holding were recovered near their bodies, police said.

Holmgren also showed reporters three guns and challenged them to differentiate between real and fake guns. He then said all three weapons were fake. But regardless of whether a gun is real, someone looking at the weapon would feel threatened, Holmgren said.

Israel said the police weren't trying to defend their actions. Instead, he lamented what he believed were preventable incidents, saying most people comply with officers when asked to show their hands or drop their weapons.

"The people who are dead don't have to be dead," the deputy chief said.

"This young man had a choice to make," Israel said of Buenrostro. "He chose not to take the gun out and throw it down but to actually point it at a police officer. I'd like to get the people out there to understand - if we believe they have a gun, we're going to defend ourselves, and sometimes that means shooting at people."